Known process Control systems or Industrial Automation systems protect, control and monitor processes in industrial plants for e.g. manufacturing goods, transforming substances, or generating power. They also monitor and control extended primary systems like electric power, water or gas supply systems, or telecommunication systems, including their respective substations. An industrial automation system can have a large number of process controllers distributed in an industrial plant or over an extended primary system, and communicatively interconnected via an industrial communication system.
Substations in high and medium-voltage power networks include primary devices such as electrical cables, lines, bus bars, switching devices, power transformers and instrument transformers, which can be arranged in switch yards and/or bays. These primary devices are operated in an automated way via a Substation Automation (SA) system. The SA system includes secondary devices, so-called Intelligent Electronic Devices (IED), responsible for protection, control and monitoring of the primary devices. The IEDs may be assigned to hierarchical levels, such as the station level, the bay level, and the process level, where the process level is separated from the bay level by a so-called process interface. The station level of the SA system includes an Operator Work Station (OWS) with a Human-Machine Interface (HMI) and a gateway to a Network Control Centre (NCC). IEDs on the bay level, which may also be referred to as bay units, in turn are connected to each other as well as to the IEDs on the station level via an inter-bay or station bus serving the purpose of exchanging commands and status information.
A communication standard for communication between the secondary devices of a substation has been introduced as part of the standard IEC 61850 entitled “communication networks and systems in substations”. For non-time critical messages, IEC 61850-8-1 specifies the Manufacturing Message Specification (MMS, ISO/IEC 9506) protocol based on a reduced Open Systems Interconnection (OSI) protocol stack with the Transmission Control Protocol (TCP) and Internet Protocol (IP) in the transport and network layer, respectively, and Ethernet as physical media. For time-critical event-based messages, IEC 61850-8-1 specifies the Generic Object Oriented Substation Events (GOOSE) directly on the Ethernet link layer of the communication stack. For very fast changing signals at the process level such as measured analogue voltages or currents IEC 61850-9-2 specifies the Sampled Value (SV) service, which like GOOSE builds directly on the Ethernet link layer (Layer 2 in OSI). Hence, the standard defines a format to publish, as multicast messages on an industrial Ethernet, event-based messages and digitized measurement data from current or voltage sensors on the process level.
The patent application EP-A 2148473 relates to mission-critical industrial automation applications, such as process or drive control systems, based on a ring-type communication network with a plurality of switching nodes and operating with full duplex links. Real-Time data communication for time-critical and availability-critical automation systems calls for both seamless resiliency against faults in the network and deterministic delivery of time-critical data. Seamless tolerance against link failures in the network can be provided by giving each node two communication ports and letting the nodes send frames with identical payload over both ports, as for instance specified in the standard IEC 62439-3 Clause 5 (termed High availability Seamless Redundancy (HSR). Hence, for each message to be sent on an exemplary ring network, a source and a duplicate frame are transmitted in opposite directions, both frames being relayed by the other nodes of the ring network until they eventually return back to the originating sender node. As a consequence, network load is roughly doubled with respect to a known ring network, but the destination node will receive the data after a maximum transmission delay that equals the longest possible path of the ring. In the fault-free state, the destination node thus receives two redundant frames with the same contents, with a certain time skew due to the fact that if one frame is received directly from the neighbour node, the other frame circulates around the entire ring. The redundant frames can be identified by a sequence number hence a node can detect duplicates and only forward the earlier or first frame of the two frames to its upper layer protocols and discards the later or second frame.
A deterministic transmission of data guarantees that the data are delivered in the fault-free state with a maximum delay between the moment they are ready for transmission at the source and the moment they are received by their final destination, in spite of possible communication errors or delays. A deterministic transmission of data specifies that sufficient bandwidth is pre-allocated. Each node is expected to transmit its data with a fixed length at a fixed frequency which is defined upon configuration of the entire network thus occupying a fixed portion of the bandwidth. A propagation delay when traveling along the transmission medium as well as a forwarding delay for crossing switches and easily exceeding the propagation delay have to be added to the duration of the frame itself. Hence, an exemplary worst-case occupancy of a transmission medium connecting 5 nodes, with each node transmitting at 1 kHz frames of 2500 bits at 100 Mbit/s plus 5 μs of propagation time (resulting in 30 μs duration per frame), is at least equal to 5×30×10−6×103=15%.
In real-time communication networks, such as IEC 61850-9-2, the traffic is carried by periodically sent R-frames, where “R” stands for “regular” or “real-time”. By contrast, non-critical, soft real-time data are subject to relaxed timing constraints, and are expected to meet the delivery delays only with a certain probability. This non-critical traffic is carried by “S-frames”, where “S” stands for “sporadic” or “soft-time”. The minimum network capacity for R-frames can be estimated at engineering time, for instance from the Substation Configuration Description (SCD) file specified in IEC 61850. The SCD file indicates how often the time-critical frames SV and GOOSE should be transmitted and which size they have, and includes a communication subsection defining the topology of the network and the number of switches. However, the communication delays for R-frames may exceed the numbers computed at engineering time, since collisions take place within a switch and the R-frame, in spite of its higher priority, must be queued while an S-frame is being transmitted over the same medium.
Deterministic communication is traditionally provided by limiting the production rate in each node so that an overload is excluded, and in addition by regulating the traffic so that collisions are avoided, using for instance one of 1) a Time Division Multiplex Access (TDMA) slot allocation triggered by a common clock established before useful data communication can take place; 2) a central master polling the source addresses in a predefined sequence, as e.g. specified in IEC 61735; or 3) a master frame introduced by a master device that will be filled and read by all contributing devices, such as used be EtherCat. The latter two options are difficult to achieve where frames can be duplicated and may take different paths. Hence, the first method has been retained for HSR, providing a simple clock-based TDMA can get around configuring a central master or sending a dedicated master frame or other token.
PROFINET (Profibus.com) distinguishes between non real-time (NRT), real-time (RT), and Isochronous real-time (IRT) data. PROFINET IRT uses a combination of IEEE 1588, TDMA, and specialized switches to achieve hard real-time performance. Upon configuration of the network, the network topology and the data specified from each device are analyzed and cycles with a real-time period and a non-real-time period are defined. Each IRT device is assigned a time slot to communicate during the real-time period. Because all the IRT devices are synchronized using IEEE 1588, only one IRT device at a time sends packets. Switches with special ASICs buffer packets sent from non-IRT devices during the real-time period and transmit them during the non-real-time period.